Communication devices, such as cellular telephones, allow voice communications over wireless communications networks. Such devices have become commonplace in today's world. In recent years, efforts have been made to leverage existing wireless communication systems, so that not just voice information may be communicated, but so that data from, for example, a portable (i.e. laptop) computer or personal digital assistant (PDA) may also be transmitted over the wireless networks. One result of such efforts is the General Packet Radio Service (GPRS) standard, which provides a packet switched data overlay of the Global System for Mobile Communications (GSM) wireless cellular system.
GPRS utilizes a “timeslot” principle, whereby each radio frequency (RF) carrier signal is divided into eight time slots. This allows the GPRS system to provide eight communication channels per carrier signal. By using several timeslots in parallel, data may be transmitted faster. In theory, when all eight timeslots are used, GPRS allows a maximum transmit speed of 171.2 kilobits per second (kbps). In practice, however, this data rate is not possible, and devices are categorized by the number of timeslots the devices are able to use to transmit (TX) and receive (RX) data. For example, a Class 8 device uses one TX slot and four RX slots. A Class 10 device, by comparison, uses two TX slots and four RX slots, meaning that it may transmit data bursts two times as fast a Class 8 device.
Because data from laptop computers and PDAs cannot be directly communicated over wireless networks, interface devices are necessary to gain access to the wireless network. Such interface devices include means for formatting the data in accordance with system standards (e.g. GPRS) and means for modulating a radio frequency (RF) signal by the data to be transmitted, so that the data can be transmitted wirelessly over the wireless network.
Various standards have been developed that set forth both electrical specifications and form factor requirements for interface devices of the type described above. One standard that is in common use today is the PCMCIA (Personal Computer Memory Card International Association) standard. PCMCIA is an organization, consisting of some five hundred companies, which has developed a standard for small, credit card-sized devices, called PC Cards. Although originally directed at adding memory to portable computers, the PCMCIA standard has been expanded several times and is now applicable to many types of devices other than memory. There are three types of PCMCIA PC Cards, designated as Type I, Type II and Type III. All three types have the same rectangular size (85.6 by 54 millimeters), but each differs in thickness. A Type II card can be up to 5.0 mm thick, and is the type that is used for the interface devices described above. Such interface devices, when in the form of a PC Cards, are commonly referred to as PC Card wireless modems.
PC Cards plug into a PCMCIA slot designed into a laptop computer or PDA. FIG. 1 shows a wireless data terminal 10 comprising a host computer 100 (e.g., a laptop computer or PDA) and a PC Card wireless modem 102. The PC Card wireless modem 102 includes an antenna 104 for transmitting/receiving radio frequency (RF) signals to/from a remote device over a wireless network. The PC Card wireless modem 102 also includes various input/output (I/O) and power and ground terminals 106, which are arranged according to the PCMCIA standard. The host computer 100 communicates with the PC Card wireless modem 102, via a PCMCIA interface 108, when terminals 106 are connected to corresponding terminals in a PCMCIA slot 110 of the host computer 100. The PCMCIA interface 108 also provides connections for supplying power from the host computer power supply (i.e. battery) to the PC card wireless modem 102, when terminals 106 are connected to corresponding terminals in the PCMCIA slot 110 of the host computer 100. This allows the PC Card wireless modem 102 to derive all of its power from the battery of the host computer 100. Hence, the PC Card wireless modem does not require its own dedicated power supply. FIG. 2 shows a conceptual diagram of a laptop computer 200 with a PC Card wireless modem 202 plugged into the PCMCIA slot 204 of the laptop computer 200.
Among other electrical specifications, the Type II PCMCIA standard specifies that the PC Card never draw more than 1 amp of current from the host power supply at any one time. Unfortunately, the power amplifier (PA) in the RF section of the PC Card wireless modem requires large currents, especially during burst transmits. This current requirement increases as the number of TX slots used by the PC Card wireless modem increases. Due to the difficulty in satisfying the PCMCIA maximum current draw standard, some PC Card wireless modem designs include an onboard “super capacitor,” which is connected in parallel with the host power supply. The super capacitor lends itself as a current source during high current demand burst transmits, thereby supplementing the current provided by the host supply. In this manner the 1 amp PCMCIA maximum current draw specification can be satisfied.
Another standard of recent interest is the CompactFlash Plus (CF+) standard. The CF+ standard is an extension of the original CompactFlash (CF) standard, which was originally developed by the CompactFlash Association (CFA) for the purpose of providing small, lightweight storage devices for mobile products. The CF+ specification expands the concept beyond flash data storage to include I/O devices such as, for example, wireless modems. An attractive feature of the CF+ standard is that the form factor is smaller (about the size of a matchbook) than the form factor of a PCMCIA card, which as explained above is about the size of a credit card. A drawback from a design standpoint, however, is that the CF+ standard specifies that, at 95% of 3.3 volts, only 500 mA of peak current may be drawn from the host power supply. This is about half the allowable current draw permitted by the PCMCIA specification. Unfortunately, because many wireless modems require much more current than 500 mA, especially during burst transmits, this specification cannot be complied with. For example, a Class 10 wireless modem requires more than 1.3 amps in a two timeslot transmission configuration. Whereas attempts at achieving compliance to the CF+ standard have been made using a super capacitor, similar to that described above, these attempts have failed since even with the addition of a super capacitor the wireless modems draw currents from the host power supply that exceed the 500 mA maximum current draw limit. Accordingly, use of a super capacitor alone is not an acceptable solution to achieving specification compliance.